Development processes for the preparation of electronic and optoelectronic devices are typically serial in nature. Thin films with desired crystal structure can be grown using a template, i.e., using substrates that match the lattice constant of deposited film. For example, for preparation of DRAM capacitors, a high-κ rutile phase of TiOx is prepared only when TiOx is deposited on a lattice matched substrate such as RuO2. However, developing materials systems or thin film stacks using templated substrates can be a very time consuming and costly process. First, a substrate film is developed that meets certain criteria, for example, exhibiting the desired crystal structure. Once the desired film has been formed, the deposited film and stack properties are evaluated. This process is slow, inefficient and costly, requiring many substrates and numerous deposition experiments to be performed serially.
High-κ materials are in increased demand for use in metal-insulator-metal (MIM) structures such as capacitors used for dynamic random access memory (DRAM) and field effect transistor (FET) gate structures. The feature sizes of next generation devices require scaling of the effective oxide thickness (EOT). For example, Barium-strontium titanate (Ba,Sr)TiO3 (BST) thin films are of interest for their potentially ultrahigh dielectric constant (κ≈300), but the BST dielectric constant is strongly dependent on thickness, and decreases to ˜16 for a 20 nm thick film on a Ru bottom electrode. BST formed on TaSiN barrier layers was amorphous, whereas high-κ is only obtained for well-crystallized materials. The crystalline form has a perovskite lattice structure. Growing BST thin films on base-metal substrates has been attempted to lower material cost, but high temperature annealing (400-750° C.) in oxidizing ambient is necessary for making BST with a high dielectric constant, and interfacial layers are introduced due to oxidation of the base-metal electrodes, resulting in a high EOT, which makes BST/base-metal stacks ineffective and undesirable. Kim et al. (“Capacitors with an Equivalent Oxide Thickness of <0.5 nm for Nanoscale Electronic Semiconductor Memory,” 2010 Adv. Funct. Mater. 20, 2989-3003) provide an extensive discussion of why dielectric constant can be a strong function of layer thickness. The interfacial layers mentioned above are believed to create a “dielectric dead layer” such that the region does not function as the desired dielectric medium. The dead layer is believed to be caused by a variety of effects including dislocations, secondary phases, interdiffusion, and imperfect electrode-dielectric interfaces. The defects can be in the dielectric, the electrode or both. Careful attention to the electrode-dielectric interface can reduce the effects by minimizing crystalline defects and sharpening the interface by minimizing interdiffusion.
In another example, the high dielectric constant of rutile phase TiO2 (up to κ≈114 for films with random texture), decreased thickness dependence, and low dielectric loss make it an attractive high-κ material. However, TiO2 thin films grown by both physical vapor deposition and chemical vapor deposition usually have anatase or amorphous structures, which only have a moderately high dielectric constant (20≦κ≦40). High temperature annealing (above 700° C.) is required to transform them to the rutile phase, which requires high thermal budgets and causes structural integrity problems. Atomic layer deposition (ALD) with halide-based precursors can deposit rutile-structured TiO2, but residual halide impurity causes the films to have high leakage currents. The as-deposited rutile TiO2 films have been formed only in a few systems with substrates such as oxidized noble metals (Ru and Ir) or RuO2 and insulating single-crystal oxides (sapphire (001) and MgO (001)). These substrates have lattice planes matching rutile TiO2 that make nucleation of the rutile phase kinetically preferable to the anatase phase.
Wang et al. demonstrated that thin films of high dielectric constant (κ≈68) rutile phase titanium dioxide (TiO2) could be grown epitaxially on tin dioxide substrates. (Wang, H., et al., 2010 Electrochemical and Solid-State Letters, 13, G75-78). Atomic layer deposition at 250° C. was used with titanium(IV) tetrakis(isopropoxide) and hydrogen peroxide as precursors. The rutile TiO2 thin films were shown to have crystalline grains that match the structure and orientation of the grains in the polycrystalline rutile phase SnO2 substrates. The epitaxial relations were identified from the continuous lattice fringes across the interfaces.